Saturday , October 23 2021

Alternator viva questions part-1

Alternator viva questions part-1


Q: What is an alternator? 

Ans: The machine which converts the mechanical energy into A.C. electrical energy is known as an alternator.

Q: What are the main parts of an alternator? 

Ans: Stator, rotor, fan, slip ring, and side cover, etc. 

Q: What are the three essential elements for generating the e.m.f.? 

Ans: The source of the prime mover or say motion, magnetic field, and armature conductors, 

Q: What are the sources of prime mover generally used for the alternators?

Ans: The water turbine, steam turbine, and diesel engine.

Read: Alternator viva questions part-2

Q: How the magnetic field produces? 

Ans: The magnetic field is produced by the magnetic poles. The field can be of two types the stationary and diesel engine. 

Q: Generally which type of field is used an alternator? 

Ans: Rotating field type. 

Q: What are the main rotating parts of an alternator? 

Ans: Armature, slip rings, fun, bearing, pulley, exciter etc. 

Q: Name the stationary parts of the alternator? 

Ans: The stator winding (may be poles in case of stationary pole type construction or three phase armature winding for rotating pole type.) Side covers, rocker, and fingers, body or yoke, bed sheet, etc. 

Q: Which type of alternator is common in use? 

Ans: Rotating field type. 

Read: Alternator viva questions part-2

Q: Why stationary field-type alternators are not preferred because of design aspects? 

Ans: The space of three phase armature winding is more in stationary armature type construction, because of more circumferential area, more space for providing insulation, and winding, more space also facilitates for low dielectric strength insulation suitability, more conductors can be easily housed (increasing generated voltage) working space is more, etc. 

Q: Is there any specific advantage between load current and voltage induced?

Ans: Yes, when the armature is fixed the load current wills not create any sparking as in the case of rotating armature type construction. The sliding contact is dangerous than a fixed one. 

Alternator viva questions part-1

Q: What is the function of an exciter? 

Ans: An exciter is used to excite the field winding of the alternator for producing magnetic flux. 

Q: Will an exciter induce only a fixed voltage? 

Ans: No, the voltage induced can be changed by adding/or removing a field resistance in the field winding. 

Q: Which way does adding or removing external resistance in the field winding helps the alternator? 

Ans: If the load on the alternator is more, the voltage drop will result, hence to increase the voltage or compensate for the voltage drop, the field current has to increased causing an increase in induced voltage i.e. emf simultaneously for lowering the voltage we have to reduce the field current (when the load current is less and voltage induced is more)

Read: Alternator viva questions part-2

Q: Which method of voltage control is used?

 Ans: Field control method.

Q: Can you name the general type of D.C. generator used as an excitor?

Ans: It is a D.C. shunt generator.

Alternator viva questions part-1

Q: How will you collect the emf (Rotary armature type) or feed the current (rotary field type) from the rotor circuit?

Ans: Through collecting fingers (rotary armature) add the brushes.

Q: What is the utility of the fan?

Ans. The fan is provided for air circulation, the heat is produced and the alternator is kept cool.

Q: Can you give some idea about the instruments mounted on the panel board of a three-phase alternator?  

Ans: The panel board is supposed to have three ammeters MI type for A.C. measurement, three voltmeters (MI type) for line voltage measurement (a rotating switch may also be provided to note the line voltage in that case one switch and one voltmeter or ammeter is sufficient), a D.C. voltmeter, a power factor meter, a wattmeter, an energymeter, a D.C. voltmeter, a D.C. ammeter, two circuit breakers, two field rheostats for alternator and excitor. 

Q: What is synchroscope? 

Ans: It is instrument used for synchronizing the alternators. 

Read: Alternator viva questions part-2

Q: What is the basic principle behind this instrument? 

Ans: Relative speed of two rotating magnetic fields. 

Q: How these fields are obtained? 

Ans: One because of the already running supply and other by the e.m.f. induced of the incoming machine. 

Q: Which load is known for its magnetizing characteristics?

Ans: Leading power factor load current has magnetic characteristic.

Alternator viva questions part-1

Q: Lagging power factor load is known for its behavior as….? 

Ans: Demagnetizing nature.

Q: What is the working principle of the alternator? 

Ans: The alternator works on the principle of faraday’s laws of electro-magnetic induction.“Whenever a conductor cuts the line of force (magnetic flux) an e.m.f. is induced in that conductor.”

Q: How the alternators are classified? 

Ans: These are classified in accordance with:

  1. The field construction 
  2. The number of poles 
  3. The types of field excitation 
  4. The types of pole construction

Read: Alternator viva questions part-2

Q: Name the types of alternators according to the field? 

Ans: a) Stationary field type.

         b) Rotating field type.

Q: What is the difference between the rotary and stationary field type construction? 

Ans: The alternator in which the magnetic field is stationary is called the stationary field and those in which field is rotating are called rotating field type, 

Q: What are the advantages of a rotating field-type alternators? 


  1.  Only two slip rings are required
  2. Winding can be properly insulated and housed.
  3. More number of armature conductors can be placed as more space is available in stator than rotor.
  4. Winding (armature) is stationary so less possibility of faults.
  5. Less maintenance cost, 

Q: According to the pole construction, what are the types of alternators?


  1. Salient pole type
  2. Smooth cylindrical type. 

Q: What do you mean by salient pole type? 

Ans: The poles are projected on the rotor. These types of rotor have more diameter than the axial length. 

Q: Can this type of rotor be run at first speed? 

Ans: No. 

Q: What are the smooth cylindrical type poles?

 Ans: In this type, the alternator has less diameter and more axial length. In this construction, the projection of poles is totally avoided and the rotor is of smooth cylindrical nature. 

Read: Alternator viva questions part-2

Q: How many poles are generally designed in smooth rotor construction?

Ans: Two or four. 

Q: Is this construction suitable for high speed? 

Ans: Yes 

Q: Can you tell the number of poles designed for salient pole type rotor construction? 

Ans: Six pole to 48 poles. 

Q: How the alternators are classified according to the number of phases?

Ans: Single phase, three phase etc.

Q: How the field winding is excited an alternator? 


  • a) By means of a separate battery,
  • b) By means of a D.C. generator mounted on the same shaft. 

Q: Which types is preferred? 

Ans: Generator mounted on the same shaft.

Q: What is the main advantage of this excitor? 

Ans: Economy of space. 

Read: Alternator viva questions part-2

Q: Is there any disadvantages also? 

Ans: Yes, The exciter has to run on same space as that of alternator. 

Q: In general the speed of these D.C. shunt generators is not so fast so what about the mechanical size of this excitor and the normal D.C. generator?

 Ans: In comparison these are made smaller in size. 

Q: How the poles are fixed in the rotor of an alternator having speed less than 25 m/sec? 

Ans: Bolted with rotor. 

Q: If the speed is higher than 25 m/sec how these are fixed? 

Ans: With the dovetailed joints with the rotor.

Q: What is the em.f. equation of the alternator? 


E=2.Kf.Kd.Kc.∅.f.z voltz


E=emf generated in volts
Kf=Fom factor i.e 1.11 

Kd= Distribution factor 

Kc = Coil span factor

∅= flux in wb 

f = frequency in c/s

z = Number of conductors 

Q: What is the frequency of generation if an alternator is running at 1500 r.o.m. having four poles? 

Ans: 50 c/s. 

Read: Alternator viva questions part-2

Q: How the frequency can be calculated? 

Ans: f=(P x N)/120

f = frequency of generation in c/s

N = Speed in r.p.m.

P = Number of poles. 

Q: An alternator having four poles is running at 1800 rpm. What will be the frequency of induced e.m.f.? 

Ans: 60 c/s.

Q: A 25 cfs alternator running at 1500 rpm is producing 450 V, 50 c/5 supply, now if the speed is reduced to 1200 rpm, how much voltage it will produce? 

Ans: it will give 360 V. 

Q: But what about the frequency? 

Ans: The frequency of the generated e.m.f. will drop down because the frequency and speed both are directly proportional to each other. 

Q: A 10 pole alternator will run at what speed for providing 50 c/s supply?

Ans: 600 rpm. 

Read: Alternator viva questions part-2

Q: An 84 poles hydro turbine is running to generate 50 c/s supply, what is the speed? 

Ans: 71.42 rpm approximately. 

Q: If the speed of an alternator is kept constant and the field current is changed, what will be the effects on frequency of induced emf and magnitude of induced emf? 

Ans: The frequency of generation will be same but the emf induced will be changed correspondingly, 

Q: What will happen if the field current is kept constant and the speed of alternator is increased? 

Ans: The emf induced will be continuously increased but primarily the frequency of emf will increase which will be of great concern. Hence this procedure is not encouraged. 

Q: Which generator is generally used for excitor and how the induced voltage is fed to the alternator? 

Ans: D.C. shunt generator and the voltage is fed through the leads and terminals. 

Q: How the frequency is governed? 

Ans: By governing the speed of prime mover. 

Q: On what factors does the frequency of an alternator depend? 

Ans: Poles and speed.

Read: Alternator viva questions part-2

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